Nanostructures, such as carbon nanotubes and nanowires, have been used in broad applications ranging from nanocomposites to nanoelectromechanical systems (NEMS).
Characterization of electrical transport properties of nanostructures can be carried out using various metrics. One of the most frequently used is resistivity, which measures the current response of the nanostructure with an applied voltage or vice-versa. Two direct methods exist for this characterization, namely, two-terminal and four-terminal-Kelvin characterization.
Reports of two and four-contact measurements of resistivity exist for many nanostructures, in particular for nanowires. In the two-terminal technique, an electrical contact is made on each end of a slender specimen. By making current (I) flow from one end to the other, and measuring the voltage (V) developed across the terminals, for various values of the current, the equivalent resistance of the nanostructure can be obtained from the slope of the V vs. I curve. From this, the resistivity of the nanostructure can be obtained from its geometry. Alternatively, the same can be done by applying a voltage and measuring the current.
However, this method has a caveat; namely, no matter the way the contacts are made, there is an associated resistance (Rc) to them. As a result, the test current (I) will make appear a potential difference across them (Vc), which adds to the actual potential difference in the nanostructure (Vn). However, because the measurement is recording the total potential difference (V), the slope of a V vs. I curve will reveal the resistance of the specimen plus contacts, not just the one of the specimen. As a conclusion it can be said that the measurement will reflect the true properties of the nanostructure (Rn) only if the resistance of the contacts is very close to zero or very small compared to the resistance of the nanostructure (Rc<<Rn).
Unfortunately, because contacts to nanostructures are complex and difficult, this condition is hardly ever fulfilled. In many cases the contact is made through electron-beam-induced deposition (EBID) of platinum (Pt) or Ion-beam induced deposition (IBID) of the same metal. However, because these depositions have a relatively large percentage of amorphous carbon [reference 5], the resistance of this deposition is large compared to that of pure platinum and consequently cannot be ignored. On other cases, contact is made through metal electrodes, such as gold [reference 6]. This can be advantageous if the specimen has a similar work function to that of the metal electrode, but that is not always the case, resulting in formation of non-ohmic contacts.
An object of the present invention is to employ the four-terminal Kelvin technique in a microelectromechanical device (MEMD) or system (MEMS) to eliminate any influence of the contacts in the measurement. In this technique, a current is injected into the specimen through two contacts made in the outermost ends of the sample. In between these two contacts, another pair of terminals is connected and the voltage developed across them is measured. Because the measurement of voltage relies on an instrument with very high input impedance, the current flowing through the voltage meter will be almost zero. Thus, the influence of the resistance of the contacts is eliminated because no matter their value, the slope of the curve of V vs. I will always be the resistance of the portion of the specimen between the inner terminals.